ESTRO 2020 Abstract book

S410 ESTRO 2020

Conclusion Observers scored the image quality of MR higher than CBCT and had significantly greater confidence using MR to perform direct LN registrations. T1 DIXON outperformed the T2 TSE MR sequence in providing more accurate registrations. The T2 TSE shows fat and LNs as bright contrast. This might make it difficult to differentiate the edges of mediastinal LNs, whereas the T1 DIXON provides a fat suppressed image which may aid LN identification in this scenario. However no difference was seen between MR and CBCT.. This work suggests MR may be useful in increasing the accuracy for set-up correction during RT for LA NSCLC. Further evaluation of its potential role is needed. PD-0674 The role of radiation therapists in the area of online and adaptive treatment on the MR Linac G. Sikkes 1 , E.N. De Groot - van Breugel 1 , N.G.P.M. Vissers 1 , G.H. Bol 1 , S.L. Hackett 1 , P.S. Kroon 1 , B. Van Asselen 1 , I.M. Jϋrgenliemk-Schulz 1 1 UMC Utrecht, Radiation Oncology Department, Utrecht, The Netherlands Purpose or Objective The role of radiation therapists (RTTs) in multidisciplinary treatment teams can change when innovations are implemented into clinical use. Here we present how the new online adaptive treatment workflows as present in the 1.5T MR Linac (MRL) (Unity, Elekta, Veenendaal) influenced RTTs work and responsibilities at our institute using SBRT for lymph node oligometastases (LNO) as an example. Material and Methods Our institute started in August 2018 with MRL treatments with LNO in the pelvis being the first clinical site. Patients were treated with fractionated SBRT (5x7Gy) using online planning on the actual anatomy as provided within the Elekta Adapt to Shape workflow (ATS). This process includes MRI scanning, image fusion, contour propagation, delineation, planning and plan QA. A multidisciplinary team of 1 radiation oncologist (RO), 1 medical physicist (MP) and 3 RTTs is responsible for the individual treatment. Prior to start treating patients with the MRL, RTTs are trained for MR scanning and online adaptive workflows. They further specialize in two profiles with competence for: 1. image registration and contouring; 2. treatment planning/plan evaluation/plan QA. To further optimize the workflow and reduce the workload for RO and MP, additional steps have been taken. A training program was started to achieve full competence for target en organ at risk delineation by RTTs. Contouring by RTTs started under on-site supervision by RO and ended with RTTs contouring with supervision on demand. For treatment planning and plan evaluation and QA, RTTs started with on-site supervision by MP and also end up with supervision on

Results 24 LNs distributed between hilar (station 10-11) and mediastinal regions (1-7) were assessed. Fig 2 shows the distribution of individual LN σV values for each imaging modality. The mean σV for CBCT was not significantly different than either MR Seq1 (0.22 vs 0.27, p= 0.09) or Seq2 (0.22 vs 0.20, p= 0.47). However the mean σV for Seq 1 was significantly greater than Seq2 (0.27 vs 0.20, p= 0.04). Compared to CBCT the mean confidence scores for Seq1 (2.5 vs 1.8, p<0.001) and Seq2 (2.7 vs 1.8, p<0.001) were both significantly better. Seq2 had a significantly greater mean score than Seq1 (p=0.041). Regarding image quality, compared to CBCT, both Seq1 (2.66 vs 1.75, p<0.001) and Seq2 (2.74 vs 1.75, p<0.001) had significantly greater scores. No difference was seen between Seq1 and Seq2 (p=0.5).